This invention relates to a joint prosthesis for permanent anchorage in the bone tissue of a joint in the human body, for instance a knee joint.
Various methods are already known for dealing with knee joint destruction and other joint diseases or malformations in joints in the human body through prosthetic surgical intervention.
The joints which have been objects of such prosthetic surgical intervention are primarily the knee joint, hip joint, elbow joint, shoulder joint, foot joint and finger joint. The present invention is not confined to any one of these joints but will hereinafter be described mainly in conjunction with knee joints, since it is there that there is a pronounced need to extend the indication range for knee joint replacement so that considerably younger patients than at the present time could be offered a solution to their knee problems.
The disease-caused defects concerned are rheumatic knee joint diseases and wear injuries that in the first instance affect joint cartilage and in the second instance underlying bone tissue which is worn away and gives rise to a varying degree of joint defect. A defect of this nature may be localized and affect only one or two or the three compartments of the knee joint as in the majority of cases of knee joint wear, but it may also be generalized as in rheumatic disease, when both the medial and the lateral knee joint compartments and also the femuro-patellar joint are involved in the same destructive process. In localized knee joint destruction, if the destruction is limited solely to the medial or lateral knee joint compartment, only this part is provided with a prosthesis, in which case it is customery to speak of a demiprosthetic procedure (half-joint prosthesis). If two or three compartments are involved in the arthrosis, it will usually be a matter of a total prosthesis. This is almost always the case with rheumatic diseases.
The disease-caused defect found in connection with a joint-destroying disease or wear injury is often extended in conjunction with prosthetic surgery to make room for the prosthesis and to create a mechanical lock between prosthesis and bone. The contour of the skeleton end is thus removed to fit more or less exactly into the for example multicornered box formed by the surrounding prosthesis. The surgical defect thus encroaches upon intact joint skeleton.
The need of stabilization between the parts of the prosthesis varies from one defect situation to another. If the joint ligament apparatus is defective it is prior art knowledge to provide the opposed articulating joint replacements with a coupling in the form of a hinge or piston mechanism which replaces the stabilization normally obtained through ligaments and cruciate ligaments. Such stabilized prostheses are however rather voluminous and increase the surgical defect.
In the majority of cases, around 90 percent of all knee joint destructions, the ligaments and cruciate ligaments are nevertheless so intact that no stabilized prosthesis is needed. All the same, for a moderate but not grave ligament injury some type of retention between the parts of the prosthesis may be necessary and can then be built into the prosthesis design. For example it is known in the art to cup the articulation socket of the prosthesis and to adapt the joint head thereto. A prosthesis of this type is designated as constrained. In a constrained prosthesis there is a good congruence between joint surfaces, implying good stability and less compression stress per unit of area but also increased friction between the joint surfaces and less possibility of movement in certain planes, e.g. translation. A constrained prosthesis retards movements and thus gives rise to strains in mobility planes that are not permitted by the prosthesis and this implies that the prosthesis absorbs constrained forces. This strain will be transmitted to the transition between prosthesis and bone. Experience shows that constrained prostheses have a very high loosening frequency which probably is due to the absorption of constrained forces which are thus transmitted to the transition between prosthesis and bone.
Although a constrained prosthesis can be made less voluminous than the stabilized prosthesis the disadvantage of the high loosening probability nevertheless remains. In recent times, therefore, a new type of prosthesis has been developed, namely a semi-constrained or non-constrained prosthesis. These prostheses have a surface geometry which in principle is characterised by a rounded joint ball which rests against a relatively flat joint socket. These types of prosthesis obviously impose rather high demands on the ligaments and cruciate ligaments for their stabilization but in recompense the ligaments and musculature absorb the strain to which the prosthesis is subjected in all motional planes and the prosthesis will thus not brake the strain on account of its structure so that the constrained forces acting on the transition between prosthesis and bone will be reduced. The frequency of loosening is also significantly lower with this type of prosthesis than with the earlier type, but today the follow-up time for semi-constrained and non-constrained prostheses is far too short to permit any definite conclusions to be reached with regard to the long-term forecast for loosening frequency. One thing, however, is perfectly clear with regard to this type of prosthesis and that is that the contact surface between the parts of the prosthesis is reduced which leads to a greater load with compression forces per unit of area, which involves a risk of mechanical wear. In addition, exacting demands are imposed on the ligaments since the prosthesis is very little stabilized in its own structure.
In summary, it may therefore be said that a constrained prosthesis is exposed to a greater risk of loosening than a non-constrained prosthesis, but the risk of loosening is by no means excluded in that the prosthesis is non-constrained. Moreover, there is a greater risk of mechanical wear problems with non-constrained prostheses.
The problem of mechanical wear has naturally also been studied. On the basis of 15 years of clinical experience quite a lot is known about the behaviour of metals, plastics and ceramics as joint replacement materials. Tests have been performed on replacement of both joint socket and joint head with the same material. Different material combination possibilities have also been tried. Steel against steel, for example, has been found to be unfavourable, whereas ceramic against ceramic has proved to be a favourable combination in view of resistance to mechanical wear. This latter combination, however, appears to be advantageous only in ball-and-socket joints. Generally, metal against polyethylene is considered to be an acceptable combination, and it is probable that this particular combination will remain in the forefront of interest for at least another ten years. Polyethylene is biologically inert and has a beneficial elastic deformation that dampers peak forces. Polyethylene is also highly resistant to permanent deformation if the component has a thickness of at least 6 mm and if it is metal-supported in relation to the bone surface. The issue being discussed today therefore is not whether or not the joint surface of the tibial component in a knee joint shall consist of polyethylene but is rather a matter of which metal should be combined with the polyethylene as a replacement material for the joint surface of the thigh bone end. No really certain alternative is, however, afforded here. Further studies of mechanical wear resistance, corrosion resistance, toxic effect etc. are required.
Attempts have been made in various ways to increase the anchorage stability of the joint substitute in the bone tissue. More than twenty years of experience has now been gained of cement fixation (with methyl acrylate) of joint replacements, particularly in hip joints, and it is known that the forecast in the 10-year perspective is reasonable as regards clinical stability of prostheses inserted under optimal conditions. With a greater space of time after the reconstruction the risk of loosening nevertheless increases significantly and this can also be discerned at a relatively early stage. Radiological loosening, in fact, precedes the clinical loosening and can occasionally be seen several years before typical loosening discomfort is experienced by the patient. In view of the doubtful prognosis for cement-fixed prostheses in the long term one is generally undisposed to use such replacements in young patients who not only can be expected to need the prosthesis for a very long time but also subject the bone-prosthesis relation to greater strain on account of a higher level of activity.
It is also known in the prior art to anchor joint replacements without cement. Freeman introduced his technique for cementless fixation of knee prostheses in 1976 and since then other designs such as PCA, Laskin, N.J., the Galante prosthesis and others have been presented. It is still all too early to arrive at any conclusions about the results of effects to fix knee joint replacements without bone cement but one may nevertheless assert that there is very little evidence to support lasting unchanged stabilization of the above types of prosthesis to underlying bone. All the signs indicate that most, if not all, of the types of prosthesis mentioned hereinbefore appear to be connective tissue-anchored, i.e. surrounded by connective tissue that separates the prosthesis from the bone tissue. A connective tissue zone of this nature tends to grow under continued load. This leads to loosening. With regard to the occurrence of such a connective tissue zone, reference may be made to an article by Gerald A Lord et al, "An Uncemented Total Hip Replacement", Clinical Orthopaedics and Related Research No. 141, June 1979.
From the above it is evident that there are several artificial knee joints on the market, but none of these has been shown to establish a long-lasting stable anchorage to the host skeleton.